In recent years, lasers have been employed in a wide variety of surgical procedures. For certain laser wavelengths, silica based optical fibers are suitable for delivering the laser energy from the source to the treatment site. Much effort has been expended in developing fiber optic probes for use in various types of surgeries.
Initial development efforts by the assignee herein were directed to designing a probe which could survive in the harsh environment created when high energy laser pulses are used to ablate relatively hard tissue such as cartilage and bone. More specifically, when the probe is used to deliver high energy pulses (e.g. in excess of one joule per pulse), the delivery end of the probe is subjected to heat and debris generated at the treatment site which can destroy the probe. Various design approaches have been developed to minimize damage to the delivery end of the probe during the treatment procedure. Details of the types of structures intended to prevent damage to the delivery end of the probe during a surgical procedure can be found in U.S. Pat. No. 5,257,989, issued Nov. 2, 1993, and U.S. Ser. No. 905,125, filed Jun. 23, 1992, both of which are assigned to the same assignee herein and incorporated by reference.
The designs described in the above cited references have been so successful in minimizing damage to the delivery end of the probe that it became possible to contemplate reusing the probes in second and subsequent surgical procedures. Of course, prior to reusing a probe with another patient, the probe needs to be sterilized. Therefore, additional efforts were made to design a probe which could be repeatedly sterilized. Such a probe must be able to withstand the high pressure, high temperature steam encountered in an autoclave.
To achieve this goal, the materials forming the probe were carefully selected. In addition, the internal structure of the probe was designed to remain stable during temperature cycling encountered in an autoclave. Details of this improved design are set forth in U.S. patent application Ser. No. 08/016,768, filed Feb. 11, 1993, assigned to the same assignee herein and incorporated by reference.
By using the design approaches described in the above cited references, probes were developed that could be sterilized and reused a number of times. However, even with the developments described above, a small percentage of fibers experienced failure upon reuse. The observed failure mechanism included overheating of the coupler connecting the probe to the laser device which can lead to catastrophic damage.
Various efforts were made to diagnosis and address this problem. In the belief that damage was occurring to the input face of the fiber, this input face was polished to increase its strength. While polishing the input face of the fiber resulted in a noticeable reduction in the observed failure rate, the subject invention disclosed below was developed to essentially eliminate these failures.
While studying the failure mechanism described above, the inventor herein discovered that the exposure of the input face of the fiber to the cycling of the pressurized steam occurring during sterilization in the autoclave enhanced and accelerated the formation of cracks in the silica glass at the fiber surface. It had been previously reported that water molecules in the presence of cracks in glass can accelerate the breakdown of bonds. (See, "The Fracturing of Glass," Michalske and Bunker, Scientific American, December, 1987, pages 122-129.) As can be appreciated, the high pressures generated in an autoclave can force steam molecules into any microscopic cracks present in the fiber, accelerating the break down of atomic bonds. It is also believed that when the autoclave is rapidly depressurized, turbulence is created forcing debris into the input face of the fiber thereby increasing the damage. These cracks and other imperfections lead to breakdown of the fiber during use.
When the fiber optic probe is used in a surgical procedure, the input coupler is connected to the operating laser source such that the output of the laser source is focused onto the input face of the fiber. Typically, the diameter of the focal spot on the input face is about one-half the size of the diameter of the fiber to insure that all the light energy from the laser is coupled into the fiber. Due to this concentrated focusing of the input light, the power density on the input face is quite high. If microcracks are present in the input face, a portion of the light is scattered, causing heating and then melting of the fiber which sharply decreases the amount of light being coupled into the fiber. At this point, the scattered light will heat and burn the coupler and the probe will have to replaced.
This type of failure mechanism is not observed at the delivery end of the probe in the present designs. It is believed that the overheating problem is less acute since the power density at the delivery end is much lower than at the input face. The power density is much lower at the delivery end because as the beam propagates along the fiber, it expands such that by the time it exits the fiber, the beam diameter will match the diameter of the fiber.
Accordingly, it was desirable to develop an approach which would minimize the damage to the input face of the fiber while still allowing the probe to be sterilized.